Additions of Silyl Enol Ethers to β‐Formyl Esters – a Chelate‐controlled Synthesis of the Pheromone (+)‐Eldanolide

Chelate‐controlled Mukaiyama reaction of silyl enol ethers 2a, 2b, 2c , and 2d with β‐formylcarboxylate 1 in the presence of TiCl 4 provided γ‐lactones 3a, 3b, 3c , and 3d in very good yields and with good to excellent trans selectivities. The use of BF 3 , SnCl 4 , and SnBr 4 as Lewis acids afforded 3a with moderate cis selectivity thus excluding chelate control. Starting with ( S )‐ 1 TiCl 4 ‐promoted reaction with 2b , we obtained optically active γ‐lactone 3b which was efficiently converted into the pheromone (+)‐eldanolide in two steps. The enantiomeric excess of 87% proved that the sequence proceeds without detectable racemization.